Two-fluid simulations of driven reconnection in the Mega-Ampere Spherical Tokamak

TL;DR

This study uses 2D fluid simulations to analyze magnetic reconnection during plasma start-up in the Mega-Ampere Spherical Tokamak, revealing effects of geometry and two-fluid physics on flux-rope merging.

Contribution

It provides new insights into the physics of flux-rope merging in spherical tokamaks by incorporating two-fluid effects and toroidal geometry in simulations, aligning with experimental observations.

Findings

Resistive MHD flux-ropes enter sloshing regime at eta<1E-5.

Three regimes identified in Hall-MHD depending on current sheet width.

Simulated density profiles match Thomson scattering measurements in MAST.

Abstract

In the merging-compression method of plasma start-up, two flux-ropes with parallel toroidal current are formed around in-vessel poloidal field coils, before merging to form a spherical tokamak plasma. This start-up method, used in the Mega-Ampere Spherical Tokamak (MAST), is studied as a high Lundquist number and low plasma-beta magnetic reconnection experiment. In this paper, 2D fluid simulations are presented of this merging process in order to understand the underlying physics, and better interpret the experimental data. These simulations examine the individual and combined effects of tight-aspect ratio geometry and two-fluid physics on the merging. The ideal self-driven flux-rope dynamics are coupled to the diffusion layer physics, resulting in a large range of phenomena. For resistive MHD simulations, the flux-ropes enter the sloshing regime for normalised resistivity eta < 1E-5. In Hall-MHD three regimes are found for the qualitative behaviour of the current sheet, depending on the ratio of the current sheet width to the ion-sound radius. These are a stable collisional regime, an open X-point regime, and an intermediate regime that is highly unstable to tearing-type instabilities. In toroidal axisymmetric geometry, the final state after merging is a MAST-like spherical tokamak with nested flux-surfaces. It is also shown that the evolution of simulated 1D radial density profiles closely resembles the Thomson scattering electron density measurements in MAST. An intuitive explanation for the origin of the measured density structures is proposed, based upon the results of the toroidal Hall-MHD simulations.